Weighted SAW reflector gratings for orthogonal frequency coded SAW tags and sensors

ABSTRACT

Weighted surface acoustic wave reflector gratings for coding identification tags and sensors to enable unique sensor operation and identification for a multi-sensor environment. In an embodiment, the weighted reflectors are variable while in another embodiment the reflector gratings are apodized. The weighting technique allows the designer to decrease reflectively and allows for more chips to be implemented in a device and, consequently, more coding diversity. As a result, more tags and sensors can be implemented using a given bandwidth when compared with uniform reflectors. Use of weighted reflector gratings with OFC makes various phase shifting schemes possible, such as in-phase and quadrature implementations of coded waveforms resulting in reduced device size and increased coding.

The patent application is a divisional application of U.S. patentapplication Ser. No. 11/508,674 filed on Aug. 23, 2006 which claims thebenefit of priority to U.S. Provisional Patent Application No.60/711,278 filed on Aug. 25, 2005.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The subject invention was made with government support under NASAGraduate Student Research Fellowship Program Grant No. NGT10-52642. Thegovernment has certain rights in this invention.

FIELD OF THE INVENTION

This invention relates to signal coding and, in particular, toapparatus, systems, devices and methods for generating, distributing,processing and detecting orthogonal frequency coding for surfaceacoustic wave and silicon tags and sensors for transmission of sensoridentification and information using weighted reflector gratings.

BACKGROUND AND PRIOR ART

The surface acoustic wave (SAW) sensor offers advantages in that it iswireless, passive, small and has varying embodiments for differentsensor applications. Surface acoustic wave (SAW) sensors are capable ofmeasuring physical, chemical and biological variables and have theability to operate in harsh environments. In addition, there are avariety of ways of encoding the sensed data information for retrieval.Single sensor systems can typically use a single carrier RF frequencyand a simple device embodiment, since tagging is not required. In amulti-sensor environment, it is necessary to both identify the sensor aswell as obtain the sensed information. The SAW sensor then becomes botha sensor and a tag and must transmit identification and sensorinformation simultaneously.

Known SAW devices include delay line and resonator-based oscillators,differential delay lines, and devices utilizing multiple reflectivestructures. Single sensor systems can typically use a single carrierfrequency and a simple coding technique, since tagging is not required.However, there are advantages of using spread spectrum techniques fordevice interrogation and coding, such as enhanced processing gain andgreater interrogation power.

The use of orthogonal frequencies for a wealth of communication andsignal processing applications is well known to those skilled in theart. Orthogonal frequencies are often used in an M-ary frequency shiftkeying (FSK) system. There is a required relationship between the local,or basis set, frequencies and their bandwidths which meets theorthogonality condition. If adjacent time chips have contiguous localstepped frequencies, then a stepped chirp response is obtained. See S.E. Carter and D. C. Malocha, “SAW device implementation of a weightedstepped chirp code signal for direct sequence spread spectrumcommunication systems”, IEEE Transactions on Ultrasonics,Ferroelectrics, and Frequency control, Vol. 47, July 2000, pp. 967-973.

Orthogonal frequency coding is a spread spectrum coding technique thathas been successfully applied to SAW tags and sensors as described in D.Puccio, D. C. Malocha, D. Gallagher, J. Hines, “SAW sensors usingorthogonal frequency coding,” Proc. IEEE International Frequency ControlSymposium, 2004, pp. 307-310. OFC offers several advantages over singlefrequency SAW tags and sensors including enhanced processing gain,increased range using chirp interrogation, and improved security. Inaddition, OFC relies on the use of several frequencies makingsimultaneous sensing and tagging possible in multiple sensorenvironments. OFC SAW devices are implemented using reflectors withperiodicities chosen to match the chip frequency of interest.

As a result, an OFC SAW device contains a series of reflectors whosecenter frequencies correspond to the OFC signal of interest. In the caseof high reflectivity or narrow bandwidth materials, it is desirable tocontrol the reflection and transmission characteristics of eachreflector. The present invention uses weighted reflectors as a method ofcontrolling the reflected and transmitted SAW energy. In addition,arbitrary pulse shapes can be achieved using reflector weighting.Furthermore, pulse shaping can be used inphase shift keying, such asinphase and quadrature channels in minimum shift keying (MSK).

SUMMARY OF THE INVENTION

A primary objective of the invention is to provide a new method, system,apparatus and device for applying weighting with orthogonal frequencycoding to decrease reflectively.

A secondary objective of the invention is to provide a new method,system, apparatus and device that uses weighting with orthogonalfrequency coding to allow more chips to be implemented on a device andincreases code diversity.

A third objective of the invention is to provide a new method, system,apparatus and device that uses apodized reflectors with orthogonalfrequency coding in surface acoustic wave devices in tags and sensors toincrease the number of tags and/or sensors that can be employed using agiven bandwidth.

A fourth objective of the invention is to provide a new method, system,apparatus and device that uses weighting with orthogonal frequencycoding to make various phase shifting schemes possible, such as inphaseand quadrature implementations of coded waveforms resulting in reduceddevice size and increased coding.

A first preferred embodiment of the invention provides an orthogonalfrequency coding technique for applying the orthogonal frequency codingto a surface acoustic wave device for ID tags and sensors. The tagincludes a transducer and a bank of reflector gratings on either side ofthe transducer. The reflectors are fabricated so that each reflectorproduces an orthogonal frequency corresponding to the carrier frequency.The device code is determined by the order of the orthogonalfrequencies. The reflectors are weighted reflectors designed forimplementation of orthogonal frequency coded surface acoustic wavedevices. Various reflector stopband responses are implemented byspatially weighting reflectors in a fashion similar to interdigitaltransducer apodization. The reflector gratings include cosine weightedreflector grating, apodization weighted reflectors or withdrawl weightedreflectors for the purpose of increasing code diversity wherein eachcosine reflector includes plural adjacent rectangular electrodes thatstaggered across a reflector aperture with a portion of the pluralelectrodes are connected to a first bus bar and a remaining portion ofthe plural rectangular electrodes are connected to a second bus bar toreduce group delay variations over the reflector aperture.

In a second embodiment, an orthogonal frequency coded device is producedby applying a transducer to a substrate, fabricating plural reflectorgrating on the substrate coupled with said transducer, and weightingeach of said plural reflector gratings so that the plural weightedreflectors gratings generate an orthogonal frequency coded signal. Theplural reflector gratings are shuffled to produce a different orthogonalfrequency coded signal, wherein the code is determined by the order inwhich the contiguous orthogonal frequencies are used. Weighting each ofsaid plural reflector gratings is accomplished using cosine weightedreflectors, implementing the orthogonal frequency coded device usinginphase and quadrature channels and applying pseudo noise along with theinphase and quadrature channels.

Further objects and advantages of this invention will be apparent fromthe following detailed description of preferred embodiments which areillustrated schematically in the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an example of a stepped chirp response.

FIG. 2 is an example of an OFC chip frequency response.

FIG. 3 is an example of a 7 chip OFC waveform based on the placement ofchips.

FIG. 4 is a frequency response of a 7 chip OFC device (solid line) and asingle carrier (dashed line).

FIG. 5 shows the time autocorrelation (½ length) of a single carrierBPSK (dashed line) and a 7 chip OFC (solid line) signals havingapproximately the same time length.

FIG. 6 shows the time autocorrelation (½ impulse length) of a singlecarrier PN code (dashed line) and a PN-OFC (solid line) signal having a7 chip Barker code modulating the chips of both signals.

FIG. 7 shows the frequency response of a 7 chip PN-OFC signal (solidline) and a single carrier signal (dashed line).

FIG. 8 is a block diagram of an example of an OFC SAW system accordingto the present invention.

FIG. 9 is a schematic diagram of an example of an OFC SAW sensorimplementation.

FIG. 10 shows two compressed pulses having a differential time delaybetween pulses.

FIG. 11 is a graph showing velocity verses reflector duty cycle for fourdifferent normalized metal thicknesses.

FIG. 12 is a schematic of a cosine weighted apodized reflector.

FIG. 13 shows 2-dimensional COM simulated and experimental responses ofa 24 period cosine weighted apodized reflector.

FIG. 14 is a 2-dimensional COM simulated and experimental cosineweighted reflector time domain responses.

FIG. 15 is a schematic drawing of a three chip cosine weightedorthogonal frequency coded sensor.

FIG. 16 shows simulated and experimental time domain responses of athree chip orthogonal frequency coded surface acoustic wave temperaturesensor.

FIG. 17 is a graph showing a compressed pulse response.

FIG. 18 is a 3-dimensional graph of a cosine weighted orthogonalfrequency coded surface acoustic wave sensor compressed pulses.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before explaining the disclosed embodiments of the present invention indetail it is to be understood that the invention is not limited in itsapplication to the details of the particular arrangements shown sincethe invention is capable of other embodiments. Also, the terminologyused herein is for the purpose of description and not of limitation.

The following is a list of the reference numbers used in the drawingsand the detailed specification to identify components:

200 OFC SAW system 210 tag 220 up-chirp 230 tag impulse response 240down-chirp 300 substrate 305 propogation 310 reflector gratings 320reflector gratings 330 transducer

Recently, orthogonal frequency coding (OFC) has been presented byMalocha, supra. Orthogonal frequency coding is a spread spectrumtechnique and has been shown to provide enhanced processing gain andreduced time ambiguity resulting in greater range and increasedsensitivity when compared with single carrier frequency devices. Thesensor works both as a tag and a sensor with the ability to send back“tagged” sensor information in a multi-sensor environment. The taginformation is provided by a series of reflectors which map into a knownchip sequence. The time-chip-sequence is coded by differing OFC andpseudo noise (PN) sequences. Implementation of an OFC sensor requiresreflectors having differing local carrier frequencies. In the case ofnarrow fractional bandwidths or high reflectivity (such as on LiNbO3),it is preferred to adjust the reflectivity per electrode in the variouschips.

Several steps must be taken in order to properly implement weightedreflectors. Both withdrawal weighting and apodization result instructures that contain various metalized and free surface regions. As aresult, an accurate description of SAW velocity within these regions iscritical to successful reflector design. Experimental velocity resultsas a function of normalized metal thickness and reflector duty cycle aregiven, and a discussion of the results is provided. Additionally, a twodimensional COM model has been created for analysis and design ofapodized reflectors, and experimental results are compared withpredictions for cosine weighted reflectors. Finally, experimental andsimulated responses of an OFC SAW sensor using cosine weightedreflectors are given. The responses are applied to a simulatedtransceiver and results are discussed.

It would be useful to review orthogonal frequency before discussing themethod, system apparatus and device for using orthogonal frequencycoding for surface acoustic wave identification tags and sensorsaccording to the present invention. Orthogonal frequencies are used tospread the signal bandwidth. The orthogonality condition describes arelationship between the local chip frequencies and their bandwidths. Asan example, consider the stepped linear chirp shown in FIG. 1. Sevencoherent carriers are used to generate the signal shown. Each chipcontains an integer number of carrier half cycles due to theorthogonality condition. Under these conditions, the resulting waveformis continuous. The conditions, however, do not require that the localfrequency of adjacent chips, that are contiguous in time, be contiguousin frequency. Instead, the time function of a bit provides a level offrequency coding by allowing a shuffling of the chip frequencies intime.

The chip frequency response is shown in FIG. 2. These responses are aseries of sampling functions with null bandwidths equal to 2·τ⁻¹. Inaddition, the sampling function center frequencies are separated bymultiples of τ⁻¹. Coding is accomplished by shuffling the chips toproduce signal such as shown in FIG. 3, wherein the adjacent frequenciesare not required to be sequential. The code is now determined by theorder in which the orthogonal frequencies are used. Both signals occupythe same bandwidth and the coded information is contained within thesignal phase. A more complete description of orthogonal frequency codingis given in D. C. Malocha, D. Puccio, D. Gallagher, “Orthogonalfrequency coding for SAW device applications,” Proc. IEEE InternationalUltrasonics Symposium, 2004, pp. 1082-1085, which is incorporated hereinby reference.

In the example shown in FIG. 3, the seven local chip frequencies arecontiguous in frequency but are not ordered sequentially in time and thechip weights are all unity. If the local chip frequencies were orderedhigh to low or low to high, the time sequence would be a steppeddown-chirp and up-chirp, respectively. The start of the chip carrierfrequency begins at zero amplitude, as seen in FIGS. 2 and 3, which is acondition of the orthogonality.

The OFC technique provides a wide bandwidth spread spectrum signal withall the inherent advantages obtained from the time-bandwidth productincrease over the data bandwidth. The OFC concept allows for a widebandwidth, chirp interrogation, frequency and binary coding per bit, areduced compressed pulse width as compared to a pseudo noise sequence,and a secure code. The OFC technique of the present invention can beapplied to ultra-wide-band applications since the fractional bandwidthcan exceed 20% and can be used in a multi-tag or sensor environment byusing proper coding techniques.

The given chip sequence represents the OFC for the bit. If there areJ-chips with J different frequencies in a bit, then there are Jfactorial possible permutations of the frequencies within the bit. Asignal can be composed of multiple bits, with each bit having the sameOFC or differing OFC. For the case of a signal, J-chips long and havinga single carrier frequency, the signal is a simple gated RF burst τ_(B)long. The frequency responses of a 7 bit OFC (solid line) and a singlecarrier signal (dashed line) are shown in FIG. 4, with both timefunctions normalized to unity and having identical impulse responselengths. The single carrier, shown as the dashed line, is narrowband andhas approximately 17 dB greater amplitude at center frequency, ascompared to the OFC (J=7), shown as a solid line, which has a much widerbandwidth.

The time domain autocorrelation for the signals is shown in FIG. 5 wherethe solid line represents the 7 chip orthogonal signal and the dashedline represents a single carrier signal. The peak autocorrelation isexactly the same, but the OFC compressed pulse width is approximately0.28·τ_(C), as compared with the single carrier compressed pulse widthof approximately a bit width, τ_(B)=7·τ_(C). This provides the measureof the processing gain (PG), which is the ratio of the compressed pulsewidth to the bit length, or in this case, PG=49.

In addition to the OFC coding, each chip can be weighted as ±1, giving apseudo noise (PN) code in addition to the OFC, namely PN-OFC. This doesnot provide any additional processing gain since there is no increase inthe time bandwidth product, but does provide additional code diversityfor tagging. FIG. 6 shows the autocorrelation of a 7 bit Barker codeapplied to an OFC (solid line) and a single carrier frequency (dashedline). The pseudo noise code has a compressed pulse width of 2·τ_(C), ora PG_(PN)=7 as compared PG_(PN-OFC)=49. The compressed pulse width ofthe OFC is a function of the bandwidth spread and not the pseudo noisecode; yielding comparable pulse-width and side lobes results, as shownin FIG. 5 without pseudo noise code and FIG. 6 with pseudo noise code.

FIG. 7 compares the waveforms of the frequency response of a 7-chipPN-OFC (solid line) and a single carrier pseudo noise signal (dashedline). As shown, each has approximately the same time lengths with themagnitudes normalized to the time amplitude peak of the pseudo noiseresponse. The PN-OFC has an increased processing gain and a narrowercompressed pulse peak over just the pseudo noise sequence, proportionalto the bandwidth spreading of the OFC.

In the preferred embodiment, apparatus, systems, devices and methods ofthe present invention provides an orthogonal frequency coding techniquefor SAW sensors incorporating weighted reflectors to increase the numberof chips that can be implemented, and consequently, increases the codedensity. The reflectors are weighted reflector grating that are designedfor implementation of orthogonal frequency coded surface acoustic wavesensors. By spatially weighing reflectors, a variety of reflectorstopband responses are implemented.

OFC waveforms can be employed in SAW devices using shorted periodicreflector gratings as shown in FIG. 9. Each chip of the OFC waveform isimplemented using a shorted periodic reflector grating. The gratingperiodicities are chosen so that the grating center frequenciescorrespond to the chip carrier frequencies. In order to keep the chiplength approximately constant, each grating must contain differentnumbers of electrodes as the periodicity changes. This is a directresult of the orthogonality condition. The equation used to find thegrating electrode counts is shown below.N _(j)=τ_(c) ·f _(j)  (1)This equation shows that the grating electrode count is directlyproportional to frequency. In addition, the normalized metal thicknessalso increases with frequency. Therefore, in a device fabricated with asingle metal thickness for all reflectors, the magnitude of SAWreflection for each chip will not be equal.

The device schematic shown in FIG. 9 is that of a temperature sensorwhich uses identical reflector banks on either side of a widebandtransducer. However, different free space delays are employed on eachside of the transducer. The impulse response of this device contains twoidentical approximations of the OFC signal shown in FIG. 2. Duringmatched filtering, this device produces two compressed pulses. Theseparation between the pulses is proportional to device temperature.

The sensor includes reflector banks on either side of a widebandtransducer as shown in FIG. 9. However, a different free space delay isemployed on either side of the device designated by τ₁ and τ₂. Thevarious chip amplitudes caused by the differences in SAW reflectiondescribed previously are observed. When this device is used with thetransceiver shown in FIG. 9, two compressed pulses result as shown inFIG. 10. The differential time delay between pulses provides the signal.

The tag information is provided by a series of reflectors that map intoa known chip sequence. The time-chip-sequence is coded by differentiallyOFC and PN sequences. Therefore, the implementation of OFC sensorsrequires reflectors having differing local carrier frequencies. In thecase of narrow fractional bandwidths or high reflectivity (such asLiNbO₃) it is desirable to adjust the reflectivity per electrode in thevarious chips. Due to varying system requirements, the use of weightedreflectors is required; both apodized and variable weighted reflectors.

The present invention provides apparatus, devices, systems and methodsusing wideband input transducers and plural weighted reflector gratingson either side of the input transducer. Various reflector stopbandresponses are implemented by spatially weighting reflectors in a fashionsimilar to interdigital transducer apodization. The weighting techniqueof the present invention, reflectivity is decreased allowing more chipsto be implemented and consequently, increases code diversity. As aresult, more tags and sensors can be employed using a given bandwidthwhen compared with uniform reflectors. In addition, weighted reflectorgratings makes various phase shifting schemes possible, such as inphaseand quadrature implementations of coded waveforms, reducing the devicesize and increasing coding.

Weighted reflectors can be employed using withdrawal weighting orapodization. Both techniques rely on eliminating portions of themetallized surface used in a uniform reflector. As a result, weightedreflector designs accounts for velocity perturbations in order to keepwave fronts from different sections of the reflector phase coherent. Forthis reason, reflector velocity data is extracted for YZ LiNbO3.

Reflector velocities are extracted using the following equation derivedfrom a transmission line model analogy.Vg(h/λ, a/p)=ν_(fs)[1+(a/p·Δν/ν _(fs))−(1/π·B ₀ /Y ₀ sin a/p)]  (2)Where v_(fs)=free space velocity

a/p=reflector grating duty cycle

h/λ=normalized metal thickness

This equation is a function of two independent variables, the reflectorgrating duty cycle and normalized metal thickness. The differencebetween metallized and free surface velocities, Δv, is proportional tothe normalized metal thickness. The transmission line model uses a shuntsusceptance term, B₀/Y₀, to account for stored energy at the front andback of each reflector electrode, and this term is proportional to thesquare of the normalized metal thickness.

The unknown terms in equation (2) are extracted using a test mask withseveral test structures. The test devices used a simple delay line witha reflector grating situated nearby. This embodiment is used in order toisolate just the reflector response using signal processing. The testmask contains thirty five devices each having different combinations ofreflector grating duty cycle and center frequency wavelength. Sevenwavelengths are used between 18 and 60 μm, and five reflector gratingduty cycles between 40% and 60% are employed. The mask is fabricated onYZ LiNbO3, and signal processing techniques are applied to the sweptfrequency responses to extract the reflector grating velocity data. Athree dimensional optimization is performed using the data in order toobtain the unknown terms in equation (2). The results for several metalthicknesses are shown in FIG. 11, which shows good agreement betweenmeasured and predicted results and that the curvature of each line isproportional to the normalized metal thickness. This is a direct resultof the stored energy effect which has a large influence for thickerfilms.

As described previously, reflector weighting can be accomplished usingwithdrawal weighting or apodization. Previous studies of withdrawalweighting focused on eliminating unwanted modes in resonant cavities byshaping reflector frequency responses as described in T. Omori, J.Akasaka, M. Arai, K. Hashimoto, M. Yamaguchi, “Optimisation of weightedSAW grating reflectors with minimized time delay deviation,” Proc. IEEEInternational Frequency Control Symposium and PDA Exhibition, 2001, pp.666-670 for example. For OFC tag and sensor applications, the timedomain response is of primary concern, and is better approximatedthrough apodization.

In an apodized reflector, each electrode covers a portion of the beamaperture defined by a sampled time window. As an example, FIG. 12 showsa schematic of a sixteen period cosine weighted reflector. The firsteight electrodes are connected to the bottom bus bar, and the rest areconnected at the top. This is done in order to reduce group delayvariations over the reflector aperture. In order to simulate such adevice, a two dimensional COM model was developed. The model is designedto segment the beam aperture into uniform tracks as shown in FIG. 12.The sum of each track's swept frequency response yields the overallreflector response.

When designing apodized reflectors, special care is taken to ensurephase coherence of the reflected waves from different tracks. Forexample, consider an incident wave on the left side of the reflectorshown in FIG. 12. A portion of the wave is reflected from track A, andafter a short delay, another portion is reflected from track B. Thisdelay is implemented using a free surface which has a slightly highervelocity than within the electrodes. As a result, if no correction ismade, the reflected waves from track A and B have a small phasedifference. The initial free space delays must be extended slightly inorder to keep the reflected waves coherent. The resulting electrodes aremade up of several smaller rectangles that are staggered across theaperture as shown.

Using the apodization technique described, a weighted reflector wasdesigned and fabricated using a cosine window for operation on YZ LiNbO3at 250 MHz. The fabricated reflector contained 24 periods and was placednear a simple delay line. The device's swept frequency response wasobtained and signal processing was applied in order to isolate thereflector response. The device was also simulated using the2-dimensional COM model, the simulated and experimental reflectorresponses are plotted in FIG. 13. Overall, there is good agreementbetween the simulated and predicted responses. However, the experimentalresponse is nonsymmetrical and has wider bandwidth than the prediction.To gain a better understanding of the differences between the simulatedand measured reflector responses, an FFT was applied to both. In FIG.14, the time domain responses reveal that the experimental reflectorresponse is shorter than the simulated which is expected due to thedifference in their bandwidths. The errors are caused by phasedifferences of the reflected waves from each track of the reflectorwhich underlines the importance of accurate velocity information whendesigning weighted reflectors.

Implementation of an OFC SAW temperature sensor was accomplished usingcosine weighted reflectors. Previous studies have defined a set oforthogonal frequencies for uniform weighting as described in D. C.Malocha, D. Puccio, D. Gallagher, “Orthogonal frequency coding for SAWdevice applications,” Proc. IEEE International Ultrasonics Symposium,2004, pp. 1082-1085 and D. Puccio, D. C. Malocha, D. Gallagher, J.Hines, “SAW sensors using orthogonal frequency coding,” Proc. IEEEInternational Frequency Control Symposium, 2004, pp. 307-310. Using asimilar approach, orthogonal frequencies are defined for several windowtypes. Orthogonal frequencies for cosine weighting are defined as fn=2n·τ⁻¹. Using this definition, a cosine weighted OFC temperature sensorwas designed for operation on YZ LiNbO3. The sensor was implementedusing three cosine weighted reflectors as shown in FIG. 15, and occupieda 24.5% fractional bandwidth centered at 250 MHz.

The device was simulated using the 2-dimensional COM model and verifiedby experiment using devices fabricated on YZ LiNbO3. The simulated andexperimental responses have been transformed into the time domain usingan FFT, and are shown in FIG. 16. The plot shows the cosine weightedreflector responses from either side of the transducer and shows goodagreement between the measured and predicted responses.

The experimental response was then applied to a simulated transceiver; acompressed pulse response from one reflector bank is shown in FIG. 17.The autocorrelation of the matched filter is also shown for comparison.The two pulses are located one half chip length away on either side ofthe compressed pulse. These pulses are undesired, and subsequentcalculations have shown that the level of these responses can besignificantly reduced by implementing the device using inphase andquadrature channels along with PN coding. The OFC sensor was tested overtemperature between 10° C. and 100° C. and the resulting compressedpulse responses are shown in FIG. 18. Using an adaptive matched filter,the pulse amplitude remains constant as temperature is varied.

The OFC SAW system 200 block diagram is shown in FIG. 8. The SAW tag 210is interrogated with a linear stepped up chirp 220 possessing the sametime length and bandwidth as the tag impulse response 230. For a givenpeak amplitude, the chirp provides increased power over a givenbandwidth as compared to a simple RF tone burst. A noise-like tagresponse signal 230 is returned from the identification tag 210. Sinceorthogonal frequencies are used, the intersymbol interference isdrastically reduced when compared with a conventional PN sequence. Aband-limited version of the tag's impulse response results after a downchirp 240 is applied. The signal is then match filtered to produce acompressed pulse.

The OFC apparatus, systems, devices and methods of the present inventionare readily applied to SAW sensing applications. An example of a SAWdevice according to the present invention is shown in FIG. 9. The deviceincludes a wideband input transducer and multiple weighted reflectorgratings 310 and 320 and operates in the differential mode using thebank of weighted reflector gratings 310 and 320 on either side of thetransducer 300. The device receives 305 an orthogonal interrogationsignal, and in response, transmits 305 an orthogonal frequency codedsignal.

In summary, the present invention provides apparatus, methods systemsand devices for the use of weighted reflectors in orthogonal frequencycoded SAW tags and sensors. A review of OFC was given, and itsapplication to SAW tags and sensors was demonstrated. The importance ofaccurate SAW velocity information was discussed, and velocity profileswere extracted for YZ LiNbO3. A description of the extraction techniquewas given, and a discussion of the results was provided.

Reflector withdrawal weighting and apodization were both considered asweighting techniques, and apodization was chosen as the optimal methodfor weighting OFC SAW devices. A description was given of apodizedreflector design criteria including correcting errors due to velocityperturbations. The final apodized reflector design was optimized toreduce group delay variations across the beam aperture and ensure phasecoherence of the reflected waves from each uniform section of thereflectors. In addition, a two dimensional COM model was developed inorder to properly simulate the apodized reflector designs. Theapodization design criteria were then applied to cosine weightedreflectors, and experimental devices fabricated on YZ LiNbO3 werecompared with simulated responses generated by the 2-D COM model. Thesimulated and experimental reflector responses were very similar;however, subsequent analysis revealed small errors due to phasedifferences of the reflected waves from each uniform track of thereflector. These errors revealed the importance of accurate materialvelocity information when designing weighted reflectors. Lastly, an OFCSAW temperature sensor was implemented using cosine weighted reflectors.

The 250 MHz OFC SAW sensor was fabricated on YZ LiNbO3, and its sweptfrequency response agreed well with 2-D COM model predictions. Thesensor was then tested over temperature and the responses were appliedto a simulated transceiver which uses an adaptive matched filter. Usingthe adaptive matched filter, results were given showing that thecompressed pulse amplitudes remained constant as temperature varied.

In summary, the present invention provides a new apparatus, system,device and method for using the OFC technique using weighted reflectors,including variable and apodized reflectors, provides a weightingtechnique that allows the designer to decrease reflectively whencompared with a uniform reflector of the same length. In an orthogonalfrequency coding tag or sensor, this allows for more chips to beimplemented and, consequently, more coding diversity. As a result, moretags and sensors can be implemented using a given bandwidth whencompared with uniform reflectors.

Additionally, the present invention makes various phase shifting schemespossible, such as in-phase and quadrature implementations of codedwaveforms resulting in reduced device size and increased coding.

While the invention has been described, disclosed, illustrated and shownin various terms of certain embodiments or modifications which it haspresumed in practice, the scope of the invention is not intended to be,nor should it be deemed to be, limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

1. A system comprising: a tag for generating an orthogonal frequencycoded identification signal for identifying a device, each tag includingtwo or more banks of reflector gratings to generate the orthogonalfrequency coded signal having plural contiguous stepped orthogonalfrequencies within a bandwidth and a transducer coupled with the two ormore banks of reflectors and an adaptive matched filter coupled betweenthe at least two banks of reflectors for filtering the orthogonalfrequency coded signal, wherein a pulse amplitude of the orthogonalfrequency coded signal is approximately constant with a varyingoperating temperature; and a transceiver for transmitting an orthogonalinterrogation signal to said tag and for receiving said orthogonalfrequency coded identification signal generated by the tag in responseto said interrogation signal.
 2. The system of claim 1, wherein each ofthe two or more banks of reflectors comprises: plural reflectors forgenerating said plural contiguous stepped frequencies for saididentification orthogonal frequency coded identification signal; and atag transducer for receiving said orthogonal interrogation signal atsaid tag and transmitting an orthogonal coded signal generated by saidtag in response to said orthogonal interrogation signal.
 3. The systemof claim 2, wherein said plural reflectors comprises: plural spatiallyweighted reflector gratings to produce plural reflector stopbandresponses.
 4. The system of claim 1, wherein said tag comprises: pluralidentification tags, each of said plural identification tags comprising:plural reflectors for generating a different orthogonal frequency codedidentification signal having plural contiguous frequencies that arecontiguous in time; and a tag transducer for receiving an orthogonalinterrogation signal at said tag and transmitting said orthogonal codedidentification signal generated by said tag in response to saidorthogonal interrogation signal.
 5. The system of claim 4, wherein eachof said orthogonal coded identification signals comprises: pluralcontiguous frequencies that are not sequential in frequency, wherein anorder of said plural contiguous frequencies identifies each of theplural identification tags.